I agree that Ubiquiti hardware adoption can be super annoying and very problematic. I do have to say that most of issues in this video are due to not handling the network first prior to plugging everything in. I can relate to the adoption issues with the hardware. I just setup two UniFi controllers on a preexisting network at home and at work last week. Instead of ranting about Ubiquiti’s broken migration tool, I'll explain how to avoid adoption issues across different subnets.
First take the config file from within the controller under the maintenance tab. If you try taking the config file from the export site setup, it will break the database every time. If you run into issues with your controller and find that you have to factory reset other hardware. Access The device on its preexisting subnet, go to its IP without the port number and Adopt the device to your controller from there. Just change the IP on the inform to match the Cloud Key. You will most likely have to access your Cloud Key on another workstation that's on its Subnet. At this point in time it should automatically be trying adopted the device to your Cloud Key and show that the Subnet of the other devices matches.
This should prevent you from having to reconfigure all of your subnets to all your different Vlans and servers or Workstations.